In winter, a battery used in photovoltaic power generators and electric vehicles is decreased in efficiency of discharge due to low temperature. For example, discharge efficiency of the battery is decreased about 50% when an external temperature is decreased below about −20° C. In addition, since mobility of ions in an electrolyte of the battery is decreased at an ambient temperature of about −10° C. or less, electric current becomes weak and output of the battery drops. In particular, when a fully charged electric vehicle (EV) is parked outside, the electrolyte is hardened due to the external temperature, thereby causing a problem in long term lifespan of the battery. Therefore, it is necessary to maintain the temperature of the battery even in winter so as to maintain lifespan and efficiency of the battery.
As a method of heating a battery, a thermoelectric element or a positive temperature coefficient (PTC) heater is used to heat air. However, various studies have been made to develop a more effective and efficient method of directly heating a battery.